Studies are in progress on cytochromes C3 from the sulfate-reducing bacteria; this cytochrome is a unique 4-electron acceptor with a negative oxidation-reduction potential. EPR potentiometric titractions have established that the four hemes from the cytochromes C3 of Desulfovibrio vulgaris and D. gigas have indeed four different and very negative oxidation-reduction potentials, ranging in values from -235 mV to -324mV, which are considerably more negative in oxidation-reduction potential values than previously believed. Mutants from Azotobacter vinelandii have been prepared by mutagenic treatment which are defective in the minor respiratory chain. Two classes are found. One class consists of reduced cytochromes C4 pluse C5 which can not reoxidized by oxygen via the terminal oxidases cytochromes A1 and O. The second class is deficient in cytochromes C4 and C5. The organism grows as well in terms of P/O ratios, cell-yield, respiratory rates, nitrogen-fixing activities as with the wild-type organism. These studies thus suggest that the minor respiratory chain is not essential to the energy requirements of the organism. Nitrite reductase from Thiobacillus denitrificans was studied by EPR spectroscopy. It was found that under reducing conditions the enzyme reduced nitrite to nitric oxide which reacted with both c- and d-hemes of the enzyme yielding an unusual EPR-detectable enzyme-NO complex which was in fact apparently diagnostic for the presence of nitric oxide, the product of the reaction.